1. Technical Field
This invention relates to a signal processing system for filtering the spectral content of a speech signal. In addition, the invention relates to a signal processing system or a coding system for coding the speech signal following the filtering to promote uniform reproduction of the speech signal.
2. Related Art
An analog portion of a communications network may detract from the desired audio characteristics of vocoded speech. In a public switched telephone network, a trunk between exchanges or a local loop from a local office to a fixed subscriber station may use analog representations of the speech signal. For example, a telephone station typically transmits an analog modulated signal with an approximately 3.4 KHz bandwidth to the local office over the local loop. The local office may include a channel bank that converts the analog signal to a digital pulse-code-modulated signal (e.g., DS0). An encoder in a base station may subsequently encode the digital signal, which remains subject to the frequency response originally imparted by the analog local loop and the telephone.
The analog portion of the communications network may skew the frequency response of a voice message transmitted through the network. A skewed frequency response may negatively impact the digital speech coding process because the digital speech coding process may be optimized for a different frequency response than the skewed frequency response. As a result, analog portion may degrade the intelligibility, consistency, realism, clarity or another performance aspect of the digital speech coding.
The change in the frequency response may be modeled as one or more modeling filters interposed in a path of the voice signal traversing an ideal analog communications network with an otherwise flat spectral response. A Modified Intermediate Reference System (MIRS) refers to a modeling filter or another model of the spectral response of a voice signal path in a communications network. If a voice signal that has a flat spectral response is inputted into an MIRS filter, the output signal has a sloped spectral response with amplitude that generally increases with a corresponding increase in frequency.
To compensate for the higher spectral output at higher frequencies of the voice signal consistent with the virtual MIRS filter, the analog communications system may include an actual low pass filter at each receiving end of a communications link to produce a flat spectral response, as opposed to a skewed spectral response. An issue arises on whether to design encoders for base stations and mobile stations that include a low pass filter to compensate for the spectral response of an analog portion of a communications network. If the analog portion affects the actual spectral response of the voice signal differently from an expected spectral response of the MIRS filter model, the resultant reproduced speech may sound odd or artificial. For example, the resultant speech may be distorted by the application of a lowpass filter that attenuates high frequency components of the voice signal that deviates from the MIRS filter model. Similarly, if no analog portion is present in the path of the voice signal, the coding performance suffers because of the presence of the superfluous low pass filter may destroy desired speech information in the high frequency region. Thus, a need exists for a system for filtering the spectral content of a signal for speech coding in a balanced manner based on the spectral characteristics of the input voice signal to be encoded.